Internet Engineering Task Force (IETF) F. Gont
Request for Comments: 6946 Huawei Technologies
Updates: 2460, 5722 May 2013
Category: Standards Track
ISSN: 2070-1721
Processing of IPv6 "Atomic" Fragments
Abstract
The IPv6 specification allows packets to contain a Fragment Header
without the packet being actually fragmented into multiple pieces (we
refer to these packets as "atomic fragments"). Such packets are
typically sent by hosts that have received an ICMPv6 "Packet Too Big"
error message that advertises a Next-Hop MTU smaller than 1280 bytes,
and are currently processed by some implementations as normal
"fragmented traffic" (i.e., they are "reassembled" with any other
queued fragments that supposedly correspond to the same original
packet). Thus, an attacker can cause hosts to employ atomic
fragments by forging ICMPv6 "Packet Too Big" error messages, and then
launch any fragmentation-based attacks against such traffic. This
document discusses the generation of the aforementioned atomic
fragments and the corresponding security implications. Additionally,
this document formally updates RFC 2460 and RFC 5722, such that IPv6
atomic fragments are processed independently of any other fragments,
thus completely eliminating the aforementioned attack vector.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc6946.
Gont Standards Track [Page 1]RFC 6946 IPv6 Atomic Fragments May 2013Copyright Notice
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Table of Contents
1. Introduction ....................................................2
2. Terminology .....................................................4
3. Generation of IPv6 Atomic Fragments .............................4
4. Updating RFC 2460 and RFC 5722 ..................................5
5. Security Considerations .........................................6
6. Acknowledgements ................................................6
7. References ......................................................7
7.1. Normative References .......................................7
7.2. Informative References .....................................7
Appendix A. Survey of Processing of IPv6 Atomic Fragments by
Different Operating Systems ............................8
1. Introduction
[RFC2460] specifies the IPv6 fragmentation mechanism, which allows
IPv6 packets to be fragmented into smaller pieces such that they fit
in the Path-MTU to the intended destination(s). [RFC2460] allows
fragments to overlap, thus leading to ambiguity in the result of the
reassembly process, which could be leveraged by attackers to bypass
firewall rules and/or evade Network Intrusion Detection Systems
(NIDS) [RFC5722].
[RFC5722] forbids overlapping fragments, specifying that when
overlapping fragments are detected, all the fragments corresponding
to that packet must be silently discarded.
As specified in Section 5 of [RFC2460], when a host receives an
ICMPv6 "Packet Too Big" message advertising a "Next-Hop MTU" smaller
than 1280 (the minimum IPv6 MTU), it is not required to reduce the
assumed Path-MTU, but must simply include a Fragment Header in all
subsequent packets sent to that destination. The resulting packets
Gont Standards Track [Page 2]RFC 6946 IPv6 Atomic Fragments May 2013
will thus not actually be fragmented into several pieces but will
just include a Fragment Header with both the "Fragment Offset" and
the "M" flag set to 0 (we refer to these packets as "atomic